Make Variant Information Required

I have recently set up a product with variants that allows us to have a customer enter their account number before sending us a payment. We can then post the payment to the proper account, since we have the account number. now I want to provide some basic validation to verify that the customer has not left the Account number variant text field blank.

After browsing google and this forum, I found a link to http://www.elated.com/articles/form-validation-with-javascript/ , which seems promising. However, I cannot seem to make it work, and I think it is due to the fact that the "Add to Cart" button is running a javascript, not performing a post operation. Either way, I added a name to the form in the e-junkie button code, and then adapted the script in the example linked above to validate the os0 variable to verify that it is not blank.